This invention relates to RF (including microwave) interconnections among layers of assemblies of multiple integrated circuits, and more particularly to interconnection arrangements which may be sandwiched between adjacent circuits.
Active antenna arrays are expected to provide performance improvements and reduce operating costs of communications systems. An active antenna array includes an array of antenna elements. In this context, the antenna element may be viewed as being a transducer which converts between free-space electromagnetic radiation and guided waves. In an active antenna array, each antenna element, or a subgroup of antenna elements, is associated with an active module. The active module may be a low-noise receiver for low-noise amplification of the signal received by its associated antenna element(s), or it may be a power amplifier for amplifying the signal to be transmitted by the associated antenna element(s). Many active antenna arrays use transmit-receive (T/R) modules which perform both functions in relation to their associated antenna elements. The active modules, in addition to providing amplification, ordinarily also provide amplitude and phase control of the signals traversing the module, in order to point the beam(s) of the antenna in the desired direction. In some arrangements, the active module also includes filters, circulators, andor other functions.
A major cost driver in active antenna arrays is the active transmit or receive, or T/R module. It is desirable to use monolithic microwave integrated circuits (MMIC) to reduce cost and to enhance repeatability from element to element of the array. Some prior-art arrangements use ceramic-substrate high-density-interconnect (HDI) substrate for the MMICs, with the substrate mounted to a ceramic, metal, or metal-matrix composite base for carrying away heat. These technologies are effective, but the substrates may be too expensive for some applications.
FIG. 1 illustrates a cross-section of an epoxy-encapsulated HDI module 10 in which a monolithic microwave integrated circuit (MMIC) 14 is mounted by way of a eutectic solder junction 16 onto the top of a heat-transferring metal deep-reach shim 18. The illustrated MMIC 14, solder 16, and shim 18 are encapsulated, together with other like MMIC, solder and shim assemblies (not illustrated) within a plastic encapsulant or body 12, the material of which may be, for example, epoxy resin. The resulting encapsulated part, which may be termed xe2x80x9cHDI-connected chipsxe2x80x9d inherently has, or the lower surfaces are ground and polished to generate, a flat lower surface 12ls. The flat lower surface 12ls, and the exposed lower surface 18ls of the shim, are coated with a layer 20 of electrically and thermally conductive material, such as copper or gold. As so far described, the module 10 of FIG. 1 has a plurality of individual MMIC mounted or encapsulated within the plastic body 12, but no connections are provided between the individual MMICs or between any one MMIC and the outside world. Heat which might be generated by the MMIC, were it operational, would flow preferentially through the solder junction 16 and the shim 18 to the conductive layer 20.
In FIG. 1, the upper surface of MMIC 14 has two representative electrically conductive connections or electrodes 141 and 142. Connections are made between electrodes 141 and 142 and the corresponding electrodes (not illustrated) of others of the MMICs (not illustrated) encapsulated within body 12 by means of HDI technology, including flexible layers of KAPTON on which traces or patterns of conductive paths, some of which are illustrated as 321 and 322, have been placed, and in which the various layers are interconnected by means of conductive vias. In FIG. 1, KAPTON layers 24, 26, and 30 are provided with paths defined by traces or patterns of conductors. The layers illustrated as 24 and 26 are bonded together to form a multilayer, double-sided structure, with conductive paths on its upper and lower surfaces, and additional conductive paths lying between layers 24 and 26. Double-sided layer 24/26 is mounted on upper surface 12us of body 12 by a layer 22 of adhesive. A further layer 30 of KAPTON, with its own pattern of electrically conductive traces 322, is held to the upper surface of double-sided layer 24/26 by means of an adhesive layer 28. The uppermost layer of electrically conductive traces may include printed antenna elements in one embodiment of the invention. As mentioned above, electrical connections are made between the conductive traces of the various layers, and between the traces and appropriate ones of the MMIC contacts 141 and 142, by through vias, some of which are illustrated as 36. The items designated MT0, MT1, MT2, and MT3 at the left of FIG. 1 are designations of various ones of the flexible sheets carrying the various conductive traces. Those skilled in the art will recognize this structure as being an HDI interconnection arrangement, which is described in U.S. Pat. No. 5,552,633, issued Sep. 3, 1996 in the name of Sharma.
As illustrated in FIG. 1, at least one radio-frequency (RF) ground conductor layer or xe2x80x9cplanexe2x80x9d 34 is associated with lower layer 24 of the double-sided layer 24/26. Those skilled in the art will realize that the presence of ground plane 34 allows ordinary xe2x80x9cmicrostripxe2x80x9d transmission-line techniques to carry RF signals in lateral directions, parallel with upper surface 12us of plastic body 12, so that RF signals can also be transmitted from one MMIC to another in the assembly 10 of FIG. 1.
Allowed U.S. patent application Ser. No. 08/815,349, in the name of McNulty et al., describes an arrangement by which signals can be coupled to and from an HDI circuit such as that of FIG. 1. As described in the McNulty et al. application, the HDI KAPTON layers with their patterns of conductive traces are lapped over an internal terminal portion of a hermetically sealed housing. Connections are made within the body of the housing between the internal terminal portion and an externally accessible terminal portion.
One of the advantages of an antenna array is that it is a relatively flat structure, by comparison with the three-dimensional curvature of reflector-type antennas. When assemblies such as that of FIG. 1 are to be used for the transmit-receive modules of an active array antenna, it is often desirable to keep the structure as flat as possible, so as, for example, to make it relatively easy to conform the antenna array to the outer surface of a vehicle. FIG. 2a illustrates an HDI module such as that described in the abovementioned McNulty patent application. In FIG. 2a, representative module 210 includes a mounting base 210, to which heat is transferred from internal chips. A plurality of mounting holes are provided, some of which are designated 298. A contoured lid 213 is hermetically sealed to a peripheral portion of base 212, to protect the chips within. A first set of electrical connection terminals, some of which are designated as 222a, 224a, and 226a are illustrated as being located on the near side of the base, and a similar set of connection terminals, including terminals designated as 222b, 224b, and 226b are located on the remote side of the base. FIG. 2b is a perspective or isometric view, partially exploded, of an active array antenna 200. In FIG. 2b, the rear or reverse side (the non-radiating or connection side) of a flat antenna element structure 202 is shown, divided into rows designated a, b, c, and d and columns 1, 2, 3, 4, and 5. Each location of array structure 202 is identified by its row and column number, and each such location is associated with a set of terminals, three in number for each location. Each array location of antenna element array 202 is associated with an antenna element, which is on the obverse or front side of structure 202. Each antenna element on the obverse side of the antenna element structure 202 is connected to the associated set of three terminals on the corresponding row and column of the reverse side of the antenna element array 202. Each antenna element of active antenna array 200 of FIG. 2b is associated with a corresponding active antenna module 210, only one of which is illustrated. In FIG. 2b, active antenna module 210b3 is associated with antenna element or array element 202b3. Active module 210b3 is identical to module 210 of FIG. 2a and to all of the other modules (not illustrated) of FIG. 2b. Representative module 210b3 has its terminals 222a, 224a, and 226a connected by means of electrical conductors to the set of three terminals associated with array element 202b3 of antenna structure 202. The other set of terminals of module 210b3, namely the set including terminals 222b, 224b, and 226b, is available to connect to a source or sink of signals which are to be transmitted or received, respectively. It will be clear that the orientation of module 210b3, and of the other modules which it represents, will, when all present, will extend for a significant distance behind or to the rear of the antenna element support structure 202, thereby tending to make the active antenna array 200 fairly thick. Also, the presence of the many modules will make it difficult to support the individual modules in a manner such that heat can readily be extracted from the mounting plates (212 of FIG. 2a). Also, the presence of many such active modules 210 will make it difficult to make the connections between the terminal sets of the active modules and the terminal sets of the antenna elements. The problem of thickness of the structure of FIG. 2b is exacerbated by the need for a signal distribution arrangement, partially illustrated as 290. Distribution arrangement 290 receives signal from a source 292, and distributes some of the signal to the near connections of each of the modules, such as connections 222b. 224b, and 226b of module 210b3.
A further problem with the structure of FIG. 2b is that the connections between the active module 210b3 and the set of terminals for array element 202b3 is by way of an open transmission-line. Those skilled in the art of RF and microwave communications know that such open transmission-lines tend to be lossy, and in a structure such as that illustrated in FIG. 2b, the losses will tend to result in cross-coupling of signal between the terminals of the various array elements.
A further problem with interconnecting the structure of FIG. 2b is that of tolerance build-up between the antenna terminal sets on the reverse side of the antenna element structure 202, the terminals of the modules 210, and the terminals of beamformer 290.
Improved arrangements are desired for producing flat HDI-connected structures which can be arrayed with other flat structures.
A short-circuited transmission line according to an aspect of the invention includes a monolithic, electrically conductive structure including (a) a solid center conductor having a circular cross-section about a central axis. The center conductor terminates at a first plane and has a first diameter at the first plane in a direction transverse to the central axis, and a second diameter, greater than the first diameter, at a second plane parallel to the first plane. The diameter of the center conductor tapers monotonically between the first and second diameters. The length of the center conductor is defined by the separation of the first and second planes. The monolithic structure further includes (b) a plurality of mutually identical solid outer conductors. Each one of the outer conductors has a circular cross-section about a longitudinal axis. The longitudinal axes of the outer conductors are parallel with the central axis of the center conductor. Each of the outer conductors terminates at the first plane, and has a third diameter at the first plane, and a fourth diameter, greater than the third diameter, at the second plane. The diameter of the outer conductors tapers monotonically between the first and second diameters. The outer conductors have their longitudinal axes equally spaced from each other at radii which make equal angles with adjacent radii. The monolithic structure also includes (c) a solid short-circuiting plate interconnecting the center conductor and the outer conductors at the second plane.
In a particular embodiment of the invention, the third diameter equals the first diameter, and the fourth diameter equals the second diameter, and the taper of the diameters of the center and outer conductors is linear. In another embodiment of the invention, the short-circuiting plate has a thickness no greater than the length of the center conductor. The periphery of the short-circuiting plate may be defined by a radius measured from the central axis of the center conductor, which radius is equal to the sum of (a) one of the radii plus (b) half of the greater of (i) the second diameter and (ii) the fourth diameter. In yet another embodiment, the length of the center conductor is no greater than the diameter of the dielectric insulator.
In one embodiment, a disk-like dielectric insulator encapsulates the monolithic structure. The insulator defines a central axis coincident with the central axis of the center conductor, a thickness sufficient to enclose that portion of the center and outer conductors lying between the first and second planes, and a periphery defined, at least in part, by a radius from the central axis sufficient to encapsulate the sides at the greatest taper, which is a radius which is greater than the sum of (a) one of the radii plus (b) half of the greater of (i) the second diameter and (ii) the fourth diameter. In one embodiment of the invention, the second and fourth diameters are equal, so the radius of the encapsulating insulator is equal to the radius of the circles on which the outer conductor axes lie, plus half the diameter of a conductor at the second plane. The insulator surrounds at least portions of the center and outer conductors, for insulating the center conductor from the outer conductors and the outer conductors from each other, except at the short-circuiting plate. The dielectric insulator may be either rigid or deformable, as an elastomer.
A method for producing a flat antenna array according to another aspect of the invention includes the step of affixing a plurality of microwave integrated-circuit chips to a planar support, with connections of the chips adjacent to the support. A short electrical transmission-line is procured. The electrical transmission-line includes
(i) a monolithic, electrically conductive structure which includes
(a) a solid center conductor having a circular cross-section about a central axis, and terminating in a first end at a first plane. The center conductor has a first diameter at a first plane transverse to the central axis, and a second diameter, greater than the first diameter, at a second plane parallel to the first plane. The diameter of the center conductor tapers monotonically between the first and second diameters. The length of the center conductor is defined by the separation of the first and second planes,
(b) a plurality of mutually identical solid outer conductors. Each one of the outer conductors has a circular cross-section about a longitudinal axis, and the longitudinal axes of the outer conductors lie parallel with the central axis of the center conductor. Each of the outer conductors terminates at a first end at the first plane, and the first ends of the outer conductors have a third diameter at the first plane. The outer conductors have a fourth diameter, greater than the third diameter, at the second plane. The diameter of the outer conductors tapers monotonically between the third and fourth diameters. The outer conductors have their longitudinal axes equally spaced from each other at radii which make equal angles with adjacent radii.
(c) a solid short-circuiting plate interconnecting the center conductor and the outer conductors at the second plane.
In a particular embodiment of the invention, the electrical transmission line also includes
(ii) a disk-like dielectric insulator defining a central axis coincident with the central axis of the center conductor, and a thickness sufficient to enclose that portion of the center and outer conductors lying between the first and second planes. The periphery of the disk-like dielectric insulator is defined, at least in part, by a radius from the central axis which is sufficient to enclose the all the outer conductors at their greatest diameter. This radius is no less than the sum of (a) one of the radii plus (b) half of the greater of (i) the second diameter and (ii) the fourth diameter. The insulator, when used, surrounds at least portions of the center and outer conductors in the axial direction, for insulating the center conductor from the outer conductors and the outer conductors from each other, but no electrical insulation between the conductors exists at the short-circuiting plate.
The method includes the step of applying the short transmission-line to the support with the first ends adjacent the support, and encapsulating the chips and the short transmission-line in rigid dielectric material, to thereby produce a structure including an encapsulated chip and transmission-line. At least portions of the support are removed from the encapsulated chip and transmission-line, to thereby expose at least portions of a first side of the encapsulated chip and transmission-line, including at least the connections of the chips and the first ends of the center and outer conductors of the short transmission-line. If the support lacks conductive traces, a layer of flexible dielectric sheet carrying a plurality of electrically conductive traces is applied to the first side of the encapsulated chip and transmission-line. At least one of the connections of at least one of the chips is interconnected with the first end of the center conductor of the transmission-line, and at least one other of the connections of the one of the chips is interconnected to the first ends of all of the outer conductors of the transmission-line, by way of some of the traces and through vias, to thereby produce a first-side-connected encapsulated arrangement. At least so much material is removed from that side of the first-side-connected encapsulated arrangement which is remote from the first side as will expose separated second ends, remote from the first ends, of the center and outer conductors of the transmission-line, to thereby produce a first planar arrangement having exposed second ends of the center and outer conductors of the transmission-line. A planar conductor arrangement including a plurality of individual electrical connections is applied over the first planar arrangement, adjacent the side of the first planar arrangement with exposed second ends of the center and outer conductors. The electrical connections of the planar conductor are selected so that, when the planar conductor arrangement is registered with the first planar arrangement, the electrical connections are registered with the center and outer conductors of the transmission-line. The planar conductor arrangement is registered with the first planar arrangement, and electrical connections are made between the second ends of the center and outer conductors of the transmission line of the first planar arrangement and the connections of the planar conductor arrangement.
In a particular method according to an aspect of the invention, the step of making electrical connections includes the steps of placing a compressible floccule of electrically conductive material between the second ends of each of the center and outer conductors of the transmission line of the first planar arrangement and the registered ones of the electrical connections of the planar conductor arrangement, and compressing the compressible floccule of electrically conductive material between the second ends of the center and outer conductors of the transmission lines of the first planar arrangement and the registered ones of the electrical connections of the planar conductor arrangement, to thereby establish the electrical connections and to aid in holding the compressible floccules in place. In a preferred embodiment of the invention, the method encapsulates the chips and the short transmission-line in the same dielectric material used in the dielectric disk.